The answer is having fewer neutrons than protons or electrons.
Answer:
The bottom/center of the pendulum
Explanation:
As it swings, the pendulum will have maximum potential energy at the top of its arc.
As it comes back towards the center that potential energy will convert into kinetic energy until it reaches the middle of its swing (when the pendulum is fully vertical) where all potential energy has been converted into kinetic energy.
This is when the kinetic energy is the highest
As it begins to move away from the center of its arc, that kinetic energy will convert into potential energy again, and the process repeats
Since, F = k . ∆x
Therefore, k = F / ∆x = 250 / 0.2 = 1250 N/m
(ps: convert 20 cm into 0.2 m)